Implementation of multiple pipes in C

Question

I'm trying to implement multiple pipes in my shell in C. I found a tutorial on this website and the function I made is based on this example. Here's the function

void executePipes(cmdLine* command, char* userInput) {
    int numPipes = 2 * countPipes(userInput);
    int status;
    int i = 0, j = 0;
    int pipefds[numPipes];

    for(i = 0; i < (numPipes); i += 2)
        pipe(pipefds + i);

    while(command != NULL) {
        if(fork() == 0){

            if(j != 0){
                dup2(pipefds[j - 2], 0);
            }

            if(command->next != NULL){
                dup2(pipefds[j + 1], 1);
            }    

            for(i = 0; i < (numPipes); i++){
                close(pipefds[i]);
            }
            if( execvp(*command->arguments, command->arguments) < 0 ){
                perror(*command->arguments);
                exit(EXIT_FAILURE);
            }
        }

        else{
                if(command != NULL)
                    command = command->next;

                j += 2;
                for(i = 0; i < (numPipes ); i++){
                   close(pipefds[i]);
                }
               while(waitpid(0,0,0) < 0);
        }
    }

}

After executing it and typing a command like for example ls | grep bin, the shell just hangs there and doesn't output any result. I made sure I closed all pipes. But it just hangs there. I thought that it was the waitpid that's was the problem. I removed the waitpid and after executing I get no results. What did I do wrong? Thanks.

Added code:

void runPipedCommands(cmdLine* command, char* userInput) {
    int numPipes = countPipes(userInput);

    int status;
    int i = 0, j = 0;

    pid_t pid;

    int pipefds[2*numPipes];

    for(i = 0; i < 2*(numPipes); i++){
        if(pipe(pipefds + i*2) < 0) {
            perror("pipe");
            exit(EXIT_FAILURE);
        }
    }

    while(command) {
        pid = fork();
        if(pid == 0) {

            //if not first command
            if(j != 0){
                if(dup2(pipefds[(j-1) * 2], 0) < 0){
                    perror(" dup2");///j-2 0 j+1 1
                    exit(EXIT_FAILURE);
                    //printf("j != 0  dup(pipefd[%d], 0])\n", j-2);
                }
            //if not last command
            if(command->next){
                if(dup2(pipefds[j * 2 + 1], 1) < 0){
                    perror("dup2");
                    exit(EXIT_FAILURE);
                }
            }

            for(i = 0; i < 2*numPipes; i++){
                    close(pipefds[i]);
            }

            if( execvp(*command->arguments, command->arguments) < 0 ){
                    perror(*command->arguments);
                    exit(EXIT_FAILURE);
            }
        } else if(pid < 0){
            perror("error");
            exit(EXIT_FAILURE);
        }

        command = command->next;
        j++;
    }
        for(i = 0; i < 2 * numPipes; i++){
            close(pipefds[i]);
            puts("closed pipe in parent");
        }

        while(waitpid(0,0,0) <= 0);

    }

}

Answer 1

I believe the issue here is that your waiting and closing inside the same loop that's creating children. On the first iteration, the child will exec (which will destroy the child program, overwriting it with your first command) and then the parent closes all of its file descriptors and waits for the child to finish before it iterates on to creating the next child. At that point, since the parent has closed all of its pipes, any further children will have nothing to write to or read from. Since you are not checking for the success of your dup2 calls, this is going un-noticed.

If you want to keep the same loop structure, you'll need to make sure the parent only closes the file descriptors that have already been used, but leaves those that haven't alone. Then, after all children have been created, your parent can wait.

EDIT: I mixed up the parent/child in my answer, but the reasoning still holds: the process that goes on to fork again closes all of its copies of the pipes, so any process after the first fork will not have valid file descriptors to read to/write from.

pseudo code, using an array of pipes created up-front:

/* parent creates all needed pipes at the start */
for( i = 0; i < num-pipes; i++ ){
    if( pipe(pipefds + i*2) < 0 ){
        perror and exit
    }
}

commandc = 0
while( command ){
    pid = fork()
    if( pid == 0 ){
        /* child gets input from the previous command,
            if it's not the first command */
        if( not first command ){
            if( dup2(pipefds[(commandc-1)*2], 0) < ){
                perror and exit
            }
        }
        /* child outputs to next command, if it's not
            the last command */
        if( not last command ){
            if( dup2(pipefds[commandc*2+1], 1) < 0 ){
                perror and exit
            }
        }
        close all pipe-fds
        execvp
        perror and exit
    } else if( pid < 0 ){
        perror and exit
    }
    cmd = cmd->next
    commandc++
}

/* parent closes all of its copies at the end */
for( i = 0; i < 2 * num-pipes; i++ ){
    close( pipefds[i] );
}

In this code, the original parent process creates a child for each command and therefore survives the entire ordeal. The children check to see if they should get their input from the previous command and if they should send their output to the next command. Then they close all of their copies of the pipe file descriptors and then exec. The parent doesn't do anything but fork until it's created a child for each command. It then closes all of its copies of the descriptors and can go on to wait.

Creating all of the pipes you need first, and then managing them in the loop, is tricky and requires some array arithmetic. The goal, though, looks like this:

cmd0    cmd1   cmd2   cmd3   cmd4
   pipe0   pipe1  pipe2  pipe3
   [0,1]   [2,3]  [4,5]  [6,7]

Realizing that, at any given time, you only need two sets of pipes (the pipe to the previous command and the pipe to the next command) will simplify your code and make it a little more robust. Ephemient gives pseudo-code for this here. His code is cleaner, because the parent and child do not have to do unnecessary looping to close un-needed file descriptors and because the parent can easily close its copies of the file descriptors immediately after the fork.

As a side note: you should always check the return values of pipe, dup2, fork, and exec.

EDIT 2: typo in pseudo code. OP: num-pipes would be the number of pipes. E.g., "ls | grep foo | sort -r" would have 2 pipes.

Answer 2

Here's the correct functioning code

void runPipedCommands(cmdLine* command, char* userInput) {
    int numPipes = countPipes(userInput);


    int status;
    int i = 0;
    pid_t pid;

    int pipefds[2*numPipes];

    for(i = 0; i < (numPipes); i++){
        if(pipe(pipefds + i*2) < 0) {
            perror("couldn't pipe");
            exit(EXIT_FAILURE);
        }
    }


    int j = 0;
    while(command) {
        pid = fork();
        if(pid == 0) {

            //if not last command
            if(command->next){
                if(dup2(pipefds[j + 1], 1) < 0){
                    perror("dup2");
                    exit(EXIT_FAILURE);
                }
            }

            //if not first command&& j!= 2*numPipes
            if(j != 0 ){
                if(dup2(pipefds[j-2], 0) < 0){
                    perror(" dup2");///j-2 0 j+1 1
                    exit(EXIT_FAILURE);

                }
            }


            for(i = 0; i < 2*numPipes; i++){
                    close(pipefds[i]);
            }

            if( execvp(*command->arguments, command->arguments) < 0 ){
                    perror(*command->arguments);
                    exit(EXIT_FAILURE);
            }
        } else if(pid < 0){
            perror("error");
            exit(EXIT_FAILURE);
        }

        command = command->next;
        j+=2;
    }
    /**Parent closes the pipes and wait for children*/

    for(i = 0; i < 2 * numPipes; i++){
        close(pipefds[i]);
    }

    for(i = 0; i < numPipes + 1; i++)
        wait(&status);
}

Answer 3

The (shortened) relevant code is:

    if(fork() == 0){
            // do child stuff here
            ....
    }
    else{
            // do parent stuff here
            if(command != NULL)
                command = command->next;

            j += 2;
            for(i = 0; i < (numPipes ); i++){
               close(pipefds[i]);
            }
           while(waitpid(0,0,0) < 0);
    }

Which means the parent (controlling) process does this:

• fork
• close all pipes
• wait for child process
• next loop / child

But it should be something like this:

• fork
• fork
• fork
• close all pipes (everything should have been duped now)
• wait for childs

Answer 4

You only need two pipes alternating like below:

typedef int io[2];

extern int I; //piped command current index
extern int pipe_count; //count of '|'

#define CURRENT 0
#define PREVIOUS 1
#define READ 0
#define WRITE 1
#define is_last_command (I == pipe_count)

bool connect(io pipes[2])
{
    if (pipe_count)
    {
        if (is_last_command || I != 0)
            dup2(pipes[PREVIOUS][READ], STDIN_FILENO);
        if (I == 0 || !is_last_command)
            dup2(pipes[CURRENT][WRITE], STDOUT_FILENO);
    }
    return (true);
}

void close_(io pipes[2])
{
    if (pipe_count)
    {
        if (is_last_command || I != 0)
            close(pipes[PREVIOUS][READ]);
        if (I == 0 || !is_last_command)
            close(pipes[CURRENT][WRITE]);
    }
}

void alternate(int **pipes)
{
    int *pipe_current;

    pipe_current = pipes[CURRENT];
    pipes[CURRENT] = pipes[PREVIOUS];
    pipes[PREVIOUS] = pipe_current;
}

Example usage:

#define ERROR -1
#define CHILD 0

void execute(char **command)
{
    static io pipes[2];

    if (pipe_count && pipe(pipes[CURRENT]) == ERROR)
        exit_error("pipe");
    if (fork()==CHILD && connect(pipes))
    {
        execvp(command[0], command);
        _exit(EXIT_FAILURE);
    }
    while (wait(NULL) >= 0);
    close_(pipes);
    alternate((int **)pipes);
}

static void run(char ***commands)
{
    for (I = 0; commands[I]; I++)
        if (*commands[I])
            execute(commands[I]);
}

I'll leave a link to a full working code for someone who needs it.

Answer 5

Building upon the idea of using a maximum of two pipes at a given time mentioned by Christopher Neylan, I put together pseudocode for n-pipes. args is an array of character pointers of size 'args_size' which is a global variable.

// MULTIPLE PIPES
// Test case:   char *args[] = {"ls", "-l", "|", "head", "|", "tail", "-4", 
0};// "|", "grep", "Txt", 0};   
enum fileEnd{READ, WRITE};

void multiple pipes( char** args){
pid_t cpid;
// declare pipes
int pipeA[2]
int pipeB[2]
// I have done getNumberofpipes
int numPipes = getNumberOfPipes;
int command_num = numPipes+1;
// holds sub array of args 
// which is a statement to execute
// for example: cmd = {"ls", "-l", NULL}
char** cmd 
// iterate over args
for(i = 0; i < args_size; i++){
  // 
  // strip subarray from main array
  //  cmd 1 | cmd 2 | cmd3 => cmd
  // cmd = {"ls", "-l", NULL}
  //Open/reopen one pipe

  //if i is even open pipeB
    if(i % 2)  pipe(pipeB);
  //if i is odd open pipeA
    else       pipe(pipeA);


  switch(cpid = fork(){
      case -1: error forking
      case 0: // child process
            childprocess(i);
      default: // parent process
           parentprocess(i, cpid);
  }
}
}
// parent pipes must be closed in parent
void parentprocess(int i, pid_t cpid){

   // if first command
   if(i == 0)  
        close(pipeB[WRITE]);

   // if last command close WRITE
   else if (i == numPipes){
       // if i is even close pipeB[WRITE]
       // if i is odd close pipeA[WRITE]
   }

   // otherwise if in middle close READ and WRITE 
   // for appropriate pipes
      // if i is even
      close(pipeA[READ])
      close(pipeB[WRITE])
      // if i is odd
      close(pipeB[READ])
      close(pipeA[WRITE])
   }

   int returnvalue, status;
   waitpid(cpid, returnvalue, status);
}
void childprocess(int i){

    // if in first command
    if(i == 0)
        dup2(pipeB[WRITE], STDOUT_FILENO);
    //if in last command change stdin for
    // the necessary pipe. Don't touch stdout - 
    // stdout goes to shell
    else if( numPipes == i){
        // if i is even
        dup2(pipeB[READ], STDIN_FILENO)
        //if i is odd
        dup2(pipeA[READ], STDIN_FILENO);        
    }
    // otherwise, we are in middle command where
    // both pipes are used.
    else{
       // if i is even
       dup2(pipeA[READ], STDIN_FILENO)
       dupe(pipeB[WRITE], STDOUT_FILENO)
       // if i is odd
       dup2(pipeB[READ], STDIN_FILENO)
       dup2(pipeA[WRITE], STDOUT_FILENO)
    }

    // execute command for this iteration
    // check for errors!!
    // The exec() functions only return if an error has occurred. The return value is -1, and errno is set to indicate the error.
    if(exec(cmd, cmd) < 0)
        printf("Oh dear, something went wrong with read()! %s\n", strerror(errno));
    }   
}

Answer 6

Basically what you wanna do is a recursive function where the child executes the first command and the parent executes the second one if no other commands are left or calls the function again.